1. Field of the Invention
The present invention relates to an A/D and D/A conversion device having an A/D conversion section and a D/A conversion section, each using a digital filter.
2. Description of the Prior Art
FIG. 1 is a block diagram showing a conventional A/D and D/A conversion device, in which indicated by 1 is an A/D conversion section and 2 is a D/A conversion section. Indicated by 3 is an analog input signal to the A/D conversion section 1, 4 is a digital output signal from the A/D conversion section 1, 5 is a digital input signal to the D/A conversion section 2, and 6 is an analog output signal from the D/A conversion section 2.
First, the internal arrangement of the A/D conversion section 1 will be explained.
Indicated by 7 is an A/D converter based on the oversampling scheme, which converts the analog input signal 3 into a digital signal 8 of a sampling frequency f.sub.s1 that is N times the maximum frequency f.sub.m of the signal band. The f.sub.s1 is assumed to be large enough as compared with the f.sub.m. Indicated by 9 is a first decimation filter which converts the digital signal 8 of f.sub.s1 into a digital signal 10 of a sampling frequency f.sub.s2 that is M times the maximum frequency f.sub.m and, at the same time, eliminates foldover noise created by the conversion. M is a number smaller than N.
Indicated by 17 is a parameter generator which determines the characteristics of the first decimation filter 9, and 11 is a second decimation filter which converts the digital signal 10 into the digital output signal 4 of a sampling frequency f.sub.s3 that is K times the maximum frequency f.sub.m and, at the same time, eliminates aliasing noise created by the conversion. K is a number smaller than M. Indicated by 18 is a parameter generator which determines the characteristics of the second decimation filter 18.
Next, the internal arrangement of the D/A conversion section 2 will be explained.
Indicated by 16 is a first interpolation filter which converts a digital input signal 5 of the sampling frequency f.sub.s3 into a digital signal of the sampling frequency f.sub.s2 and, at the same time, eliminates image components created by the conversion, 20 is a parameter generator which determines the characteristics of the first interpolation filter 16, 1.4 is a second interpolation filter which converts the digital signal 15 into a digital signal 13 of the sampling frequency f.sub.s1 and, at the same time, eliminates image components created by the conversion, 19 is a parameter generator which determines the characteristics of the second interpolation filter 14, and 12 is a D/A converter based the oversampling scheme which converts the digital signal 13 into the analog output signal 6.
The first and second decimation filters 9 and 11, and the first and second interpolation filters 16 and 14 are formed of digital filters.
Next, the operation will be explained, beginning with the A/D conversion section 1.
The analog input signal 3 entered to the A/D conversion section 1 is converted into a digital signal 8 of a very high sampling frequency f.sub.s1, which is N times the maximum frequency f.sub.m of the signal band, by means of the oversampling A/D converter 7. However, the f.sub.s1 is too high for use as the sampling frequency, and it needs to be converted to a lower sampling frequency f.sub.s3.
Although this sampling frequency conversion operation is merely to output one out of every f.sub.s1 /f.sub.s3 data items at the most (i.e., decimation), simple decimation causes the aliasing noise, resulting in a significant deterioration of the signal quality. Therefore, the signal is fed through a digital filter so as to remove the noise before decimation. The digital filter needs to meet the following two conditions.
(a) It has a sufficient attenuation in the frequency range (I.times.f.sub.s3 .+-.f.sub.m, where I is a positive integer) in which the noise exists, thereby preventing the aliasing noise. PA1 (b) It has as flat transmission characteristics as possible within the signal band. PA1 (c) It has a sufficient attenuation in the frequency range (I.times.f.sub.s3 .+-.f.sub.m, where I is a positive integer) in which the image noise exists, thereby eliminating the image noise created by interpolation. PA1 (d) It has as flat transmission characteristics as possible within the signal band.
It is very difficult to accomplish such filter characteristics by using a single-stage digital filter, because of the difference between the Nyquist frequency (f.sub.s /2) of the input signal from the maximum frequency f.sub.m of the signal band. On this account, it is a general convention to configure the digital filter in two stages made up of a first decimation filter 9 and a second decimation filter 11.
FIG. 1 is an example of the case where the digital filter is formed in two stages for the conversion of the sampling frequency. The digital signal 8 is fed through the first decimation filter 9 at a sampling frequency of f.sub.s1 so that data is decimated, and it is converted into a digital signal 10 of an intermediate sampling frequency f.sub.s2. After that, the second decimation filter 11 having more higher accuracy is used to eliminate noises remaining in the range of signal band, and data is decimated up to the final sampling frequency f.sub.s3. The first and second decimation filters 9 and 11 have their characteristics determined by the parameters provided by the respective parameter generators 17 and 18.
Next, the D/A conversion section 2 will be explained.
The D/A conversion section 2 has basically the reverse operation against the A/D conversion section 1. The A/D conversion section 1 performs the sampling frequency conversion through the decimation from f.sub.s1 to f.sub.s3, whereas the D/A conversion section 2 performs the sampling frequency conversion inversely through the interpolation from f.sub.s3 to f.sub.s1. When the sampling frequency is converted from f.sub.s3 to f.sub.s1 by the D/A conversion section, the signal is fed through a digital filter so as to remove noises outside the range of signal band, as in the case of the A/D conversion section 1. The digital filter (interpolation filter) needs to meet the following two conditions.
Since the conditions (c) and (d) are virtually identical to the conditions (a) and (b) of the foregoing decimation filter, the interpolation filter is formed of two-stage digital filters having completely same characteristics as of the decimation filter.
The digital input signal 5 of a sampling frequency f.sub.s3 is first converted into a digital signal 15 of a sampling frequency f.sub.s2 through the interpolation by the first interpolation filter 16. Subsequently, the digital signal 15 is converted into a digital signal 13 of a sampling frequency f.sub.s1 through the interpolation by the second interpolation filter 14, and thereafter is eliminated the image noise to output the digital signal 13. The digital signal 13 of a sampling frequency f.sub.s1 is converted into an analog output signal 6 by the oversampling D/A converter 12, and it is output from the D/A conversion section 2.
The conventional A/D and D/A conversion device is arranged as described above, and it necessitates four-stage digital filters, each including a parameter generator and a multiplier for the multiplication between the parameter and data, resulting in a considerably large circuit scale of the device as the whole.
The foregoing prior art is described in the article entitled "A-D/D-A Conversion Technique Based on Oversampling Scheme", in publication Nikkei Electronics, No. 458, pp. 223-231, Oct. 17, 1988, for example.